1. Field of the Invention
The present invention relates to an X-ray imaging apparatus and a control method for the apparatus.
2. Description of the Related Art
Conventionally, in the medical field, X-ray imaging apparatuses have been widely used to obtain fluoroscopic images. An X-ray imaging apparatus generally includes an X-ray irradiation unit, an X-ray detection unit, and a control processing unit. Controlling the aperture of the collimator provided for the X-ray irradiation unit makes it possible to change the spread angle of an X-ray beam. It is also possible to change the relative position between the X-ray irradiation unit and imaging unit provided in the X-ray detection unit.
More specifically, it is possible to change the position and angle of the X-ray irradiation unit and the position and angle of the imaging unit provided in the X-ray detection unit. This allows the operator to change the irradiation position and irradiation direction of X-rays in accordance with purposes.
The X-ray imaging apparatus capable of changing the spread angle, irradiation position, and irradiation direction of an X-ray beam in this manner may irradiate a region other than the imaging unit of the X-ray detection unit or a region outside the X-ray detection unit. In this case, if X-rays irradiate a region other than the imaging unit of the X-ray detection unit, an object (usually part of the body of a patient) undergoes ineffective exposure. In addition, if X-rays irradiate a region outside the X-ray detection unit, the operator and surrounding people are exposed to X-rays. For this reason, such an X-ray imaging apparatus is required to minimize X-ray irradiation outside the X-ray detection unit. There have been proposed various methods for this purpose.
For example, Japanese Patent Laid-Open No. 11-318877 has proposed a method of predicting an X-ray irradiation field by acquiring the relative position information of an X-ray irradiation unit and imaging unit and the aperture information of a collimator. According to this reference, if a predicted irradiation field includes a region other than the imaging unit, X-ray irradiation outside the X-ray detection unit is suppressed by restricting irradiation and issuing a warning to the operator.
In addition, Japanese Patent Laid-Open No. 2001-104299 has proposed an arrangement including an X-ray sensor at an edge portion of the image detection unit of an imaging unit. If the X-ray sensor detects X-rays, this arrangement performs control to stop down (reduce) the aperture of the collimator until the X-ray sensor stops detecting X-rays. According to this reference, controlling an X-ray irradiation field can suppress X-ray irradiation outside the X-ray detection unit.
Furthermore, Japanese Patent Laid-Open No. 2001-292992 discloses a technique of bringing an X-ray irradiation field close to the center of an image detection unit by generating an X-ray image using the image detection unit and predicting an X-ray irradiation field based on the generated X-ray image. If the predicted irradiation field is offset from the center of the image detection unit, an instruction to change the position of an object is issued to the operator or control is performed to change the relative position between the X-ray irradiation unit and the imaging unit. With this operation, the technique disclosed in this reference can suppress X-ray irradiation outside the X-ray detection unit.
If, however, the relative position between the X-ray irradiation unit and the imaging unit greatly changes, the relative position detection accuracy may deteriorate. The relative position detection accuracy also deteriorates due to a change with time. For this reason, in the case of Japanese Patent Laid-Open No. 11-318877, if the relative position detection accuracy deteriorates due to such a cause, the accuracy of a predicted irradiation field deteriorates. In addition, if the relative angle between the X-ray irradiation unit and the imaging unit greatly changes and the incident angle of X-rays relative to the image detection unit becomes large, the accuracy of a predicted irradiation field further deteriorates. As a result, it can occur that X-ray irradiation outside the image detection unit cannot be reliably suppressed.
In the case of Japanese Patent Laid-Open No. 2001-104299, the detection accuracy of the X-ray sensor may deteriorate due to the influences of an object. If, for example, a region having a low X-ray transmittance is imaged, the X-ray sensor may not detect X-rays in spite of the fact that an irradiation field is located at an edge portion of the image detection unit. In this case, it is impossible to reliably suppress X-ray irradiation outside the image detection unit.
In the case of Japanese Patent Laid-Open No. 2001-292992 as well, when a region having a low X-ray transmittance such as a mediastinum or an abdominal region is imaged, the prediction accuracy of an irradiation field may deteriorate. That is, it can occur that X-ray irradiation outside the X-ray detection unit cannot be properly suppressed due to the influences of an object.
For this reason, a demand has arisen for an arrangement which can reliably suppress X-ray irradiation outside the image detection unit regardless of conditions at the time of imaging (e.g. one or more of a relative position, a relative angle, a change with time, and the influence of an imaging region).